Certain spinal conditions, including a fracture of a vertebra and a herniated disc, indicate treatment by spinal immobilization. Several systems of spinal joint immobilization are known, including surgical fusion and the attachment of pins and bone plates to the affected vertebras. Known systems include screws having proximal heads and threaded shafts that may be inserted into at least two spaced-apart vertebras. Each screw includes a receiver attached over the head such that a stabilization rod can interconnect two or more receivers to immobilize the vertebras spanned by the screws. However, in these systems, the shaft is disposed through the receiver from a proximal or top side of the receiver; thus, the receiver is attached over the threaded shaft before the shaft is inserted into a vertebra.
During surgical implantation of spinal immobilization systems, the surgical site is crowded with tissue masses, sponges, and other surgical implements that may obstruct access to the sites of implantation of the threaded shafts. Further, because the receivers are necessarily larger than the heads of the screws, it can be difficult to maneuver around the receivers of prior implanted screws to implant a subsequently implanted screw near the prior implanted screws. Current spinal immobilization systems would therefore benefit from a distal loading receiver for a polyaxial bone screw including a receiver that can be attached over a generally rounded proximal head of a threaded shaft subsequent to the implantation of the threaded shaft.
Thus, the present invention helps to alleviate a lack of space at the site of implantation of a spinal immobilization system as compared to the prior art, allowing the surgeon additional freedom in locating the threaded shafts of polyaxial bone screws closer together than previously possible. The result is a significantly improved distal loading receiver for a polyaxial bone screw.